Most people are told the answer before anyone has calculated it. It is a real choice, it usually is not forced on you, and getting it right early is worth a great deal more than getting it right after the trench is dug.
An ADU can be fed from the electrical service already on the house, or it can have its own service from the utility. Both are normal. Which one suits your property depends on what the existing service can carry, what the utility will do at your address, and what the two routes cost, in that order.
Neither Seattle City Light nor PSE requires a separate service for an ADU. That surprises people, including some in the trade. The details are below, quoted from the utilities' own handbooks.
This is the distinction that causes most of the confusion, and it is worth getting straight before you talk to anyone about cost.
A service is the physical supply from the utility to the property: the drop or lateral, the service entrance conductors, the meter base, the main disconnect. A meter is how the energy gets billed. They are not the same thing, and you can have two meters on one service.
PSE will run a single service line to a meter enclosure holding two meters, one for the house and one for the ADU, where both are on one parcel billed to the same owner. Seattle City Light says that where more than one service is connected from a single drop or lateral, the building's rating is the aggregate of them. In both cases one supply, two meters.
Which matters because the load calculation follows the service, not the meters. If both dwellings sit behind one supply, that supply has to carry both, however the billing is arranged.
City Light does not allow master metering for a new duplex or multiple dwelling building. There is an explicit exception for accessory units. Their Requirements for Electric Service Connection, 2026 Edition, section 6.11:
An owner-occupied dwelling unit also containing an additional "accessory housing unit" meeting all provisions as defined in Seattle Municipal Code 23.44 and approved by the City of Seattle will be allowed to master meter.
So a permitted ADU behind an owner-occupied house may share one meter. Where an accessory unit is not separately metered, City Light bills it in the owner's name: that is section 6.8 of the same handbook, which states that all accessory dwelling units without separate metering, and any units found to have mixed or common loads, will be billed in the owner's name. The exception does not distinguish attached from detached.
It says allowed, not required. Separate metering is still available and is often what people want.
PSE's trigger is narrower and more specific than most of what gets repeated online. From their Electric Service Handbook, 2026 Edition, Single-Family Residential Projects:
Alternate Dwelling Units (ADUs) with different mailing address must be metered separately (One meter on each home).
The deciding factor is the mailing address, not whether the unit is detached, and not whether you intend to rent it. If the ADU gets its own address, expect to meter it separately. If it does not, you have a choice.
A tenant paying their own bill directly to the utility is the obvious one, and it removes an awkward conversation from every month of the tenancy. Clean separation also helps if the property is ever sold or the units are ever treated differently for tax. None of that is an electrical argument, but all of it is a real one.
Sharing the existing service is only cheaper if the existing service carries both dwellings. If it does not, you are pricing a service upgrade against a new service for the ADU, and those are closer together than most people expect.
The combined figure is not the house and the ADU added together. Two dwelling units on one service are calculated by their own method, with demand factors that do not apply to a single dwelling, and the result is often well below what a homeowner assumes from adding up appliances. That is worth having calculated properly before anyone decides the existing service is too small.
Hanco Electric runs that calculation as part of the estimate. It needs the house, the ADU as designed, and what is going in both, so it is a conversation rather than a form.
If the ADU is getting its own service, the service size calculator sizes it. An ADU is a dwelling unit in its own right, so the same NEC 220.82 method applies. Enter the ADU on its own, not the whole property. Most small cottages land on the 100 amp minimum.
If you are adding one big load to the house rather than a whole dwelling, an EV charger or a heat pump, those pages answer that directly under NEC 220.83.
For the shared-service question, call. It is the one that decides the budget and it deserves an actual calculation.
Get a free on-site estimateThat is a fork, not a dead end. A service upgrade is one route. A separate service for the ADU is another. Sometimes the answer is load management, or sequencing the work differently. Hanco Electric will price the realistic options rather than quoting only the largest one.
The utility requirements quoted on this page were read from Seattle City Light's Requirements for Electric Service Connection, 2026 Edition, and Puget Sound Energy's Electric Service Handbook, 2026 Edition, in August 2026. Utility standards are revised. Confirm current requirements with your utility for your address before relying on them. This page is general information from Hanco Electric, not a design, not a permit submittal, and not a substitute for a licensed electrician evaluating your property. The authority having jurisdiction has the final say on any load calculation.